1. Field of the Invention
The present invention relates to a motor drive, and more particularly to the motor drive having an invertor for driving a motor at variable speeds.
2. Description of the Related Art
There has been known for a multi-phase coil motor, a conventional motor drive which uses regenerative braking to decelerate the speed of the motor. The motor drive includes a d.c. power source and an invertor. The d.c. power source includes a converter and a smoothing circuit. Commercial AC power is passed through the d.c. power source and converted to d.c. power therein. The resultant d.c. power is passed through the invertor where it is inversely converted to variable frequency a.c. power, which is then used to energize the different phase coils of the motor. To slow the motor, the motor drive reduces the frequency outputted from the invertor while rendering each phase coil ON and OFF in correspondence to the subsequent decrease in speed of the motor. At times when a phase coil is rendered OFF, the current for the phase coil is diverted back to the d.c. power supply, thereby generating a regenerative current which is applied for regenerative braking.
There has been known a problem in that a great deal of noise is generated when the motor is decelerating, because the current flowing to each phase coil includes a large ripple component, for example, about 3 A against a peak value 12.5 A as shown in FIG. 1.
Two methods have been introduced to reduce this noise. In the first method a reactor is inserted in series between the invertor and each phase coil to absorb the ripple component and improve the waveform. However there has been known a problem with this first method in that the large dimensions of the reactor require increasing housing space for the motor drive. For example, a three-phase motor requires three large reactors. Such a motor drive is inappropriate for equipment and devices where compact size is paramount, or which have no surplus housing space.
In the second method, a high-speed switching element (for example, an isolation gate bipolar transistor known as IGBT) is used in the invertor to increase the switching frequency (carrier frequency). However, such high-speed switching elements are expensive, which increases the overall cost of the motor drive.